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Hello peeps,
Pulling my hair out on this on hope someone has a cure. I'm done with parts changing. Last resort till I bring to the shop as I don't give up easy and usually can fix things myself. Had a code of PO128, Thermostat or cooling temp sensor. Changed them both and code has not returned. This is not related to the running rough and bad gas mileage. I'ts much worse with AC on. I currently have no codes being displayed. This is what I have done so far:

* Remove mass air flow and cleaned it real good
* Changed spark plugs (ACdelco)
* Replaced air intake sensor
* Replaced air filter
* Replaced fuel filter
* Cleaned map sensor, new one is on the way

Any input is much appreciated. Thank You

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Location: Oregon
you're gonna need a scanner to see what is going on with your engine. For your engine you should look at the misfire counter. It will tell you what cylinders could be misfiring. You could look at your fuel trims and see if your O2 sensor matches your MAF. If the trims say lean, you could have a vacuum leak. A vacuum leak will affect your idle but may not be big enough to be noticed at rpms higher than idle.

if the misfire counter shows certain cylinders missing more than others try switching coils and plugs from the missing cylinders to other cylinders. Like if #3 is missing switch the coil to #4 and the plug with $2. Scan it again and see if the miss changed location or is it still in the same cylinder.

with the scanner see if all the temp sensors match up when the engine has sat over night and you scan it before the engine has been started. They should all be in the same area.

Check to see if the fuel pressure regulator is working correctly. check for no fuel leak at the vacuum port. 4.2L Fuel pressure should be 50 to 57 psi

tough to figure out what's going on without a scanner.

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